Further evidence of evolutionary biology at work in poultry farms comes from UK's H7N3 outbreak. (Not conclusive here, but fits evol biology – as ever with flu.)

Birds on the free range unit, however, suffered only a mild form of the flu and none died from the infection…. the virus was transported from the egg farm to the Banhams chicken farm, where it killed some 400 chickens and triggered a drop in egg production by other birds.

note also, from intensive farms:

Blood samples from birds on their farm showed that they had been exposed to the H7N3 virus as long ago as four weeks.

– during which, presumably, the virus evolved towards virulence in the "disease factories" Vets track spread of bird flu strain